The invention relates generally to semiconductor device fabrication and, in particular, to heterojunction bipolar transistors, fabrication methods for a heterojunction bipolar transistor, and design structures for a heterojunction bipolar transistor.
Bipolar junction transistors are typically found in demanding types of integrated circuits, especially integrated circuits destined for high-frequency and high-power applications. One specific application for bipolar junction transistors is in radiofrequency integrated circuits (RFICs), which are found in wireless communications systems, power amplifiers in cellular telephones, and other varieties of high-speed integrated circuits. Bipolar junction transistors may also be combined with complementary metal-oxide-semiconductor (CMOS) field effect transistors in bipolar complementary metal-oxide-semiconductor (BiCMOS) integrated circuits, which take advantage of the positive characteristics of both transistor types in the construction of the integrated circuit.
Conventional bipolar junction transistors constitute three-terminal electronic devices constituted by three semiconductor regions, namely an emitter, an intrinsic base, and a collector. Heterojunction bipolar transistors are a variant of bipolar junction transistors in which at least two of the collector, emitter, and intrinsic base are comprised of differing semiconductor materials. An NPN bipolar junction transistor includes two regions of n-type semiconductor material constituting the emitter and collector, and a region of p-type semiconductor material sandwiched between the two regions of n-type semiconductor material to constitute the intrinsic base. A PNP bipolar junction transistor has two regions of p-type semiconductor material constituting the emitter and collector, and a region of n-type semiconductor material sandwiched between two regions of p-type semiconductor material to constitute the base. Generally, the differing conductivity types of the emitter, base, and collector form a pair of p-n junctions, namely a collector-base junction and an emitter-base junction. A voltage applied across the emitter-base junction of a bipolar junction transistor controls the movement of charge carriers that produce charge flow between the collector and emitter regions of the bipolar junction transistor.
Improved device structures, fabrication methods, and design structures are needed that enhance the device performance of heterojunction bipolar transistors.